This application relates to optical fiber devices and lasers, and in particular, to single-mode fiber ring lasers.
A fiber ring laser may be constructed by using a closed fiber loop to form a ring optical resonator. The fiber ring includes at least a portion of a doped fiber as the laser gain medium which can produce an optical gain within a spectral range in response to an optical pump beam at a selected pump wavelength or within a specified pump spectral range. The fiber ring provides an optical feedback to circulate the photons at one or more wavelengths within the gain spectral range.
A laser oscillation in the fiber ring occurs at a laser wavelength within the gain spectral range when two operating conditions are met. First, the total optical gain at that laser wavelength exceeds the total optical loss in the fiber ring, and secondly, the optical phase delay associated with a round trip within the fiber ring is 360 degrees or a multiple of 360 degrees. The gain spectral range of the doped fiber, such as erbium (Er)-doped fibers, usually have a large bandwidth and hence allows multiple modes at different frequencies to oscillate at the same time. A single-mode oscillation can be achieved by selecting one particular mode to oscillate while suppressing other modes.
Various applications require single model laser oscillations. For example, wavelength-division-multiplexing (WDM) has been used to expand the capacity of a fiber communication link by simultaneously transmitting different optical waves at different WDM wavelengths. One commonly-used WDM wavelength standard is the International Telecommunication Union (ITU) standard, where the WDM wavelengths of different optical waves are required to match ITU grid frequencies. Hence, each laser transmitter needs to operate in a single mode at a designated WDM wavelength. Other applications for single-mode laser oscillations include precision spectroscopic measurements and nonlinear optical processes, among others.
Fiber ring lasers are emerging as a new generation of compact, inexpensive and robust laser sources to produce single-mode oscillations for WDM systems and other applications where a single-mode laser oscillation is desirable.
This application includes techniques and devices that use optical fibers and fiber grating devices to construct single-mode fiber ring lasers.